This invention relates to high dielectric constant type ceramic compositions, more particularly to low-temperature sintering type ceramic compositions which are suitable for multilayer capacitors having a high dielectric constant and which can be synthesized by a solid phase reaction of a composite material of oxides.
As compositions having a high dielectric constant, composite dielectric ceramics have heretofore widely been used in which barium titanate (BaTiO.sub.3) is included as a main component, and calcium zirconate (CaZrO.sub.3) and calcium titanate (CaTiO.sub.3) are further present.
These ceramics must be sintered usually at a temperature as high as 1200.degree. to 1400.degree. C., so that for multilayer capacitors including these ceramics, it is disadvantageously required to employ internal electrodes each comprising mainly an expensive noble metal such as gold, platinum or palladium which can withstand co-firing at such a high temperature.
Further, in sintering these ceramics, a great deal of heat energy of electric power, a gas or the like is necessary because of a treatment at elevated temperatures, and in consequence a used sintering furnace, sintering crucibles or the like will thus be prematurely deteriorated due to the heat, which fact disadvantageously becomes one cause for the increase in costs for the ceramic capacitors.
For this reason, there has been desired a development of compositions which permit employing inexpensive internal electrodes each composed of sliver or the like as a main component; accomplishing the sintering operation even at a temperature as low as 1000.degree. C. or less; and providing the ceramics having a large dielectric constant and a small dielectric loss.
Materials comprising Pb(Fe.sub.2/3 W.sub.1/3).sub.x (Fe.sub.1/2 Nb.sub.1/2).sub.1-x O.sub.3 (wherein x is in the relation of 0.2.ltoreq.x.ltoreq.0.5) are known which have been developed in reply to the aforesaid desire and are described in Japanese Provisional Patent Publication No. 87700/1977 (which corresonds to U.S. Pat. No. 4,078,938). Further, an attempt to add SiO.sub.2 to the aforesaid materials has been made in Japanese Provisional Patent Publication No. 15591/1978. These mateials are, however, large in temperature dependence of the dielectric loss (tan .delta.), and hence, when the multilayer capacitors are prepared by the use of such materials, they will be unpracticable.
Moreover, the multilayer capacitors which have been manufactured from the aforesaid materials are weak particularly in mechanical strength and thus have the serious practical problem that cracks and breakage are liable to occur.
An object of this invention is accordingly to provide low-temperature sintering type ceramic compositions having a high dielectric constant by which the aforementioned drawbacks are eliminated and which are high in dielectric constant, are excellent in temperature dependence of tan .delta., mechanical strength and bias dependence of the dielectric constant, and are improved in high-temperature load properties and moisture resistance.